1. Field of the Invention
The present invention relates to an electrostatographic encapsulated toner employable for producing a visible image from a latent image in an electrostatographic recording method.
2. Description of Prior Arts
As the process for fixing a toner image in a recording method such as electrostatography, there have been known three fixing processes, that is, a heat fixing process, a solvent fixing process and a pressure fixing process. Recently, the heat fixing process and the pressure fixing process, both using no solvent, are widely used from the viewpoint of the prevention of environmental pollution.
In the heat fixing process, a toner comprising a colorant bound with a binder has been conventionally employed. The same kind of toner is also employed in the pressure fixing process, but utilization of an encapsulated toner is recently proposed in the pressure fixing process.
The encapsulated toner is a toner in the form of micro-capsule prepared by enclosing a core material comprising a colorant such as carbon black and a binder with a resin shell which is rupturable by the application of pressure.
The conventional encapsulated toner is not necessarily satisfactory in various properties that are essentially required for a toner employable in the electrostatography.
In more detail, a toner employable as a developing agent in electrostatography is required to have various excellent properties such as high powder flowability, high developing efficiency, and no smearing of the surface of a photosensitive medium for producing a latent image. Further, in the case of a two-component developing process, it is necessary that the toner does not smear the surface of the employed carrier. In the pressure fixing process, high fixability, little occurrence of off-setting phenomenon on a pressure roller used in the process (namely, toner adheres to the surface of a pressure roller to stain the roller), etc. are also required for the toner.
Accordingly, the toner employed in the pressure fixing process should be satisfactory in all properties such as powder flowability, fixability to a supporting medium (e.g., paper), preservation stability of the fixed image, anti-offsetting property, and electrostatic chargeability and/or conductivity required depending upon a developing process. However, the conventional toners are not well satisfactory in the above-mentioned characteristics.
For instance, in an encapsulated toner (i.e., toner in the form of micro-capsule) containing magnetizable particles, the surface of the magnetizable particle is hydrophilic, while the core material is lipophilic. Therefore, the magnetizable particles are liable to move into the shell, while the particles should be present inside of the core material. Further portions of the particles tend to be exposed outside of the shell. For this reason, the shell is insufficient in the denseness, and accordingly it is difficult for the shell to prevent volatilization of the oily liquid from the core material (i.e., phenomenon that the oily liquid in the core portion passes through the shell to excape in storage of the encapsulated toner). If the oily liquid in the core material volatilizes, the encapsulated toner shows unsatisfactory fixability. That is, such toner is apt to deteriorate with time in the fixability even if the toner has well fixability just after the toner is prepared. For improving the above-mentioned disadvantage, U.S. Pat. No. 4,307,169 has proposed a method of coating the surface of the magnetizable particle with a polymer to make the particle lipophilic. However, the present inventor has found that thus coated polymer is easily dissolved in the oily liquid employed in the core material as a binder component. Hence, the effect given by coating the surface of the magnetizable particle with such polymer is unsatisfactory, and therefore the fixability of the encapsulated toner is not sufficiently improved. Further, the powder flowability of such toner tends to deteriorate particularly after a lapse of time.